The present invention relates to disc brakes for vehicles, and in particular to a brake disc of the disc brake arranged to reduce disc coning.
Disc brakes are increasing being used on commercial vehicles, replacing conventional drum brakes. Commercial vehicle brake discs (also referred to as “discs”) often are mounted onto axle hubs using so-called spline arrangements using a fixed or floating connection, such as taught in U.S. Pat. Nos. 6,626,273 and 7,410,036. One example a semi-floating connection is the Splined Disc® brake assembly from Bendix Spicer Foundation Brake LLC. These types of brakes typically are mounted on an axle hub having a plurality of axially-oriented splines arranged around an outer circumference of a disc-mounting region of the hub. The brake disc has corresponding radially-inward facing tabs about the inner circumference of the brake disc. The disc is mounted to the axle hub by axially sliding the brake disc onto the hub's mating splines, followed by insertion and/or attachment of a variety of fasteners, brackets, etc., as necessary per the particular splined disc's design in order to secure the brake disc against axial movement off of the hub. Alternatively, the brake disc may have a flange member extending laterally from the inner radial region at the generally parallel friction rings of the disc, an example of which may be seen in U.S. Pat. No. 4,651,851.
Very high braking energy is generated when the disc brake's caliper applies the brake pads to the brake disc to slow such heavy vehicles. During braking, the vehicle's kinetic energy is converted to heat energy, resulting in high temperatures in the brake disc.
The braking-generated heat is absorbed by and dissipated from the disc, by conduction and radiation to other portions of the brake disc and adjacent components and/or by convection via cooling air. Typically, the heat is not distributed evenly over the brake disc due to many heat transfer factors, such as natural and forced convection, radiation, lack of homogeneity in the brake disc material, and different rates of heat generation at the inner radius of the disc and the outer radius of the disc. Further, in internally-ventilated brake discs (i.e., discs having an inboard friction portion and an outboard friction portion with ventilation channels therebetween), often the configuration of the adjacent wheel and axle structure will cause uneven heat dissipation from the inboard and outboard sides, contributing to further increases in the temperature gradient between the inboard and outboard sides of the disc.
Due to gradients in temperature distribution, different regions of the disc will exhibit different amounts of thermal expansion. Where there is a temperature gradient between the inboard and outboard side plates of the disc, the resulting difference in thermal expansion will cause one side to expand more than the other side. This differential expansion manifests itself in the form “coning” of the disc, a condition in which the disc at its outer radius is axially displaced relative to the inner radius of the disc. This is an undesired phenomena, as coned discs are more susceptible to thermal juddering and heat cracking.
Coning is generally not directly addressed in most ventilated brake discs, which have friction plates that are parallel-sided, constant thickness disc plates, with constant-width air cooling channels between them. An example of a rare attempt to address coning may be seen in U.S. Pat. No. 6,116,387, which teaches an approach that varies the thickness of both disc plates simultaneously, i.e., maintaining a generally constant cooling channel width while varying the thickness of one disc plate in concert with the other disc plate. This approach maintains generally the same overall brake disc material thickness in order to placing the thickest portions of each disc plate in the radially-opposite regions where conning distortions are expected to be their worst (e.g., at the radially inner region of one disc plate and at the radially outer region of the other disc plate). Examples of this approach are shown in FIGS. 2-3 of U.S. Pat. No. 6,116,387, where the cooling channels are maintained at a constant thickness as both disc plates vary in thickness at the same rate, and the amount of disc plate material is constant at any given radius. Similarly, FIG. 1 of this document shows both disc plates' inner surface geometries being varied opposite one another such that the total material thickness is essentially constant, the maximum disc plate thickness regions are at radially opposite positions, and the cooling channels are essentially constant in width (with exception of the channels' slight widening at the extreme inner and outer radii of the channels).
In contrast to the prior art's lack of focus on coning resistance (i.e., straight-sided parallel disc plates) or constant material-width approaches as in U.S. Pat. No. 6,116,387, the present invention minimizes coning and solves other problems by achieving a more even temperature distribution between the inboard and outboard disc plates with arrangement of the material of the brake disc inboard and outboard plates and the size and shape of the ventilation channels therebetween in a differential manner, and without the need to provide for radially-inner region disc plate thinning (and consequent manufacturability difficulties).
The inventive arrangements result in volumetric and geometry differences in the material of the disc portions that alter heat reception and dissipation in different disc regions, such that the heat dissipates in a manner that minimizes temperature gradients between the inboard and outboard disc plates. In a preferred embodiment, the geometry of the surfaces of the inboard and outboard plates which face one another may be arranged to provide enhanced cooling air flow by creating a greater “nozzle effect” to draw additional cooling air through the brake disc from its inner radius toward its outer radius.
Where the highest temperatures are present at the outer diameter of the inboard and outboard disc plates (and thus the temperature differences between the two plates are relatively small), it is possible to only alter the volumetric distribution of the disc material in the region of the inner radius of the disc to adequately compensate for disc coning.
The present invention also permits lowering of stress generated by disc coning at the region of the disc's connection to the hub of the vehicle axle by minimizing angular displacement at the root of the disc's hub-interfacing teeth or attachment fasteners.
A further advantage of the present invention is optimized disc material use, potentially permitting reduction in disc mass which can lower material costs and vehicle fuel consumption, for example in cases in which material in the radially-outer region of the brake disc can be reduced due to lower projected temperatures and associated lower wear rates.
In one embodiment a brake disc includes a first side disc plate with radially-inner projections configured to engage corresponding splines on an axle hub, and a second side disc plate without radially-inner projections (for example, a side consisting primarily of a “friction ring” of material engaged by a brake pad). Typically, the first side disc plate is the inboard side of the brake disc, i.e., the side facing away from the wheel, and reaches higher temperatures than the second side facing the wheel due to there being less conductive and radiation-based heat transfer on the inboard side adjacent to the congested region of the axle hub.
In this embodiment, the axial thickness of the first side plate may be greater than that of the second side plate, and further may increase in thickness in the radially-inward direction. In such an arrangement, the internal ventilation channels between the disc plates expand in axial width in the radially-outward direction. This configuration provides a greater cooling air flow cross-section area at the outer radius of the ventilation channels as compared to the area at the inner radius of the channels, creating a “nozzle effect” which enhances the radially-outward cooling air flow and increases disc cooling.
From a temperature difference perspective, the differences in mass and geometry between the two sides of the brake disc result in reduced temperature differences between the two sides and thereby reduced coning of the brake disc. During a braking event the friction between the brake pads and their respective disc friction surfaces deposits approximately the same quantity of heat into both sides of the brake disc. In a conventional brake disc, due to the lesser heat transfer from the inboard side of the disc to its environment as compared to the relatively exposed outboard side, the temperature of the inboard side rises to a higher temperature than the outboard side of the disc.
In the present invention, the greater mass of the first side allows the heat deposited on this side of the disc to be distributed over more material than the second side, and as a result the first side does not reach as high a temperature relative to the second side as in a conventional brake disc. In addition, the increase in cooling air flow in the internal ventilation channels, as well as the greater surface area on the internal face of the disc first side exposed to the increased cooling air flow, further enhances heat dissipation from the first side and thereby further helps reduce the difference in temperature between the inboard and outboard sides of the brake disc. The decrease in temperature difference between the first and second sides of the brake disc directly leads to reduction in differences in the amounts of thermal expansion experienced by the two sides of the brake disc, and thus a reduction in the amount of coning of the disc as the two sides expand more equally.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.